In order to test the hypothesis that absence of immune response by the pregnant female is due to the production of specific anti-idiotypic antibodies, it has been necessary to develop a murine model system in which a relatively dominant idiotype can be found in a given anti-H2 immune response. In the past year we found such a dominant idiotype in BALB/C H-2dm2 anti-BALB/C humoral response. in addition, we have obtained several lines of evidence which point to the notion that anti-idiotypic antibodies do indeed regulate the ost immune response in anti H-2 response. There has not been definitive proof whether and when paternal H-2 antigen is expressed in early development. In collaboration with other groups, we examined H-2 antigen expression on mouse thophoblast clones which maintain features of trophoblasts. We found that extremely low levels of H-2 antigen is expressed in these clones. In this study, techniques to detect H-2 gene activation, protein synthesis and surface expression have been developed which will be important for the forthcoming study.